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Concentrated Solar Power 101

What is CSP?

Concentrated Solar Power (also called Concentrating Solar Power, Concentrated Solar Thermal and CSP) systems use mirrors to focus solar radiation on a thermal receiver to raise the temperature of a specially formulated heat transfer fluid (HTF) for direct use or to generate steam for hybrid power systems, for industrial processes, or to run a steam turbine for direct electrical generation.

CSP Collector Technology

Line focus collectors include linear Fresnel and parabolic trough. Point focus collectors include power tower and parabolic dish technologies. Of these technologies, parabolic trough has the longest commercial track record and the most financed, installed, and operational plants in the world. 95% of all operational CSP projects today use parabolic trough technology.

A critical and valuable feature of CSP systems is their ability to store heat for power generation at night, during cloudy weather, or to coincide with peak utility demand.

CSP Construction

CSP technology is composed of a highly reflective surface (i.e. a mirror), that redirect a large area of sunlight onto specified point. In the case of parabolic trough technology, each module unit is composed of a single parabolic mirror and a receiver. The receiver is positioned so that when the sun hits the mirror surface, the mirror redirects the sunlight into the receiver. Flowing within the receiver is a specialized Heat Transfer Fluid (HTF), such as oil or molten salt, that will absorb the thermal heat and be pumped to storage tanks or actively utilized.

Why Parabolic Trough?

30 YEARS OF EXPERIENCE

Parabolic trough systems have been in wide use for utility-grade power generation since the mid-1980’s. In recent years, other CSP geometries such as parabolic dish, linear Fresnel, and central receiver or “power tower” have received considerable attention in the marketplace to meet the need of delivering cost effective electricity. While each of these designs has particular attributes and performance characteristics that make it a potentially attractive option, parabolic troughs have been in commercial operation the longest, and been bank-financed many times.

Parabolic trough’s long history made it the ideal technology for SkyFuel to focus on innovation and cost reduction. The result was the creation of ReflecTech® Mirror Film, a highly reflective film that is ideal for line-focus collectors. Additional research and development resulted in SkyTrough®. The simple fact that SkyTrough is not composed of glass allows the mirror to slide into a pre-aligned rib system to create a parabola with optimal optical alignment, making it easier to install than conventional mirrors while maximizing the SkyTrough’s overall performance.

SkyTrough is ideal for widespread integration on a global scale, because unlike its glass counterparts, SkyTrough is durable, easy to transport, and easy to align and install.

COMMERCIALLY PROVEN

The deployment, performance and operation of commercial, utility-grade parabolic trough solar thermal power plants are well-understood and proven. In the United States, more than 350 MWe of parabolic trough plants have been operating at the cluster of nine Solar Electric Generation Systems (SEGS) facilities in California since the 1980’s. An additional 64 MWe parabolic trough plant, Nevada Solar One (NSO) has been in commercial operation in Nevada since 2007, and the Martin Solar plant in Florida since 2009. In Spain, thanks to generous government subsidies for solar power in the form of feed-in-tariffs, more than a Gigawatt of parabolic trough solar power plants was built between 2007 and 2013.

REMARKABLY EFFICIENT

The experiences at SEGS, NSO, and the plants in Spain enable the ongoing advancement of parabolic trough design and performance. All trough components, particularly the frames and support structures, the reflector surfaces, receiver tubes, sun-tracking drive and control system, and heat transfer fluid system have been evaluated and optimized over many design generations. The information gathered by the U.S. Department of Energy (DOE) and the U.S. National Renewable Energy Laboratory (NREL) from testing and experience has resulted in improved long-term performance and reduced cost for parabolic trough systems. A quarter century after first being commissioned, annual solar energy production of the first SEGS plants is higher today than when the plants were first fired up, due to increased and optimized operating procedures and component upgrades. In addition, the long operating history of parabolic trough plants means that operation and maintenance costs are predictable and well understood.

OPTIMIZED DESIGN

Building on the proven performance of parabolic trough technology while tapping into the growing body of knowledge about trough design and performance, SkyFuel addressed and advanced certain key features such as replacing the heavy, fragile glass mirrors with light, shatter-proof reflector panels made with ReflecTech and using a light, tubular space frame to boost performance and reliability while lowering cost.

MODULAR AND SCALABLE

Parabolic trough plants are inherently modular and scalable. Modularity is important for achieving low cost through high volume production of components and subsystems. The ability to simply add additional rows of troughs to scale production further makes parabolic trough power plants the premier choice for utility-grade solar power. Relative to glass-based troughs, SkyTrough can be deployed economically in very small solar fields, due to the simple assembly process.

Validation and verification at the modular unit level mitigates risk as the system is built up. The design requirements of a parabolic trough can be verified at the level of an individual module. Once the performance of a module has been vetted, the performance of an entire solar field can be reliably predicted. For SkyTrough, the basic unit of modularity is the solar collector assembly (SCA), which is comprised of eight (8) identical parabolic modules; four on either side of SkyFuel’s central OnSun™ drive and control unit.

The basic layout of a parabolic trough plant involves connecting 4, 6 or 8 solar collector assemblies in series to form a “loop.” Multiple loops are combined in parallel within the solar field heat transfer fluid circuit. Header pipes carry the heat transfer fluid from the loops to and from a steam generator to drive a steam turbine, and produce electricity.

SMOOTH AND PREDICTABLE OUTPUT

A parabolic trough solar field also has inherent “free” energy storage that, depending on the size of the plant, allows electricity production to continue well after the sun has been blocked by a cloud. Depending on the plant’s size, such thermal “momentum” can last 15 minutes or more. It means that the electrical output of the plant can remain constant (or change very gradually) during periods of partial cloud cover or as the sun sets giving utilities plenty of time to shift to other generating sources. Such inherent thermal energy storage is the result of the heat that has been collected and absorbed into the heat transfer fluid circuit. Other energy technologies that convert a renewable resource directly into electricity, such as wind and photovoltaics, can have dramatic swings in output on a minute-to-minute or even second-to-second basis during partly cloudy days, and this can present serious challenges to the transmission system. The constant and predictable output of a parabolic trough solar plant is an important benefit when considering the reliability of the power grid.

The ability to store hours of output in a dedicated thermal energy storage system allows around-the-clock generation of firm power.

FINANCEABLE

Finally, all of the parabolic trough system’s attributes make it most attractive to the banking community, which is particularly risk averse following the global financial crisis:

It is the only proven CSP technology that has been in continuous commercial operation for over 25 years.

It is the only CSP technology with completely understood performance and reliability characteristics.